It appears to me that the initial crack formed in one place, and that the H2 which was past the ignition point of 600C immediately combusted upon meeting the outside air, causing the burned spot and the melt on the SiC heater. I do not see that as evidence in itself of a hotspot — just the initial fracture point.

Perhaps at that point, the SiC shorted out where for a brief period it was running 2 kW — this might explain the heat-up/cool-down on the optical camera observed in the Bang! video.

I find it interesting that subsequently the inner pressure vessel shattered like a dropped ceramic cup into 100 pieces, and that it took the remainder of the SiC heater element with it. Was this from the hot H2 overpressure, or from the hot H20 combustion product overpressure as it tried to escape the outer tube.

I also find it interesting how the fuel powder was sintered into multiple broken cylinders.

Next time — only 1/4th material and try again. All was very interesting. Looking for some calorimetry as well.